The human heart has a number of valves for maintaining the flow of blood through the body in the proper direction. The major valves of the heart are the atrioventricular (AV) valves, including the bicuspid (mitral) and the tricuspid valves, and the semilunar valves, including the aortic and the pulmonary valves. When healthy, each of these valves operates in a similar manner. The valve translates between an open state (that permits the flow of blood) and a closed state (that prevents the flow of blood) in response to pressure differentials that arise on opposite sides of the valve.
A patient's health can be placed at serious risk if any of these valves begin to malfunction. Although the malfunction can be due to a variety of reasons, it typically results in either a blood flow restricting stenosis or a regurgitation, where blood is permitted to flow in the wrong direction. If the deficiency is severe, then the heart valve may require replacement.
Substantial effort has been invested in the development of replacement heart valves, most notably replacement aortic and mitral valves. Replacement valves can be implanted percutaneously by way of a transfemorally or transapically introduced catheter, or can be implanted directly through open heart surgery. The replacement valves typically include an arrangement of valve leaflets that are fabricated from porcine tissue or an artificial material such as a polymer. These leaflets are maintained in position by a stent or support structure.
FIG. 1A is a perspective view depicting a prior art prosthetic heart valve 8 of U.S. Pat. No. 7,682,389 (“Beith”). This valve 8 can be implanted directly and includes a stent 10 and three leaflets 30. When implanted, blood is permitted to flow from the upstream (blood inlet) end 14 towards the downstream (blood outlet) end 12, but is prevented from flowing in the reverse direction by the presence of leaflets 30. Leaflets 30 have free edges 34 located on the downstream end 12. Each leaflet 30 also has a fixed edge (or interface) 32 joined with scalloped edge portions 16a, 16b, and 16c, respectively, of stent 10. A cross-sectional plane “I” is shown that bisects the leaflet 30 joined with fixed edge 16a (located at front right). Cross-sectional plane “I” is parallel to the direction of the flow of blood and thus is vertical in FIG. 1A.
FIG. 1B is a side view of a right-side portion of valve 8 after rotation such that plane “I” is aligned with the page. From the reader's perspective FIG. 1B is viewed along a normal to plane “I.” From this view, the entirety of fixed edge 32 of leaflet 30 (which is aligned with edge 16a) lies in a flat plane and is straight with no curvature.
FIG. 1C is a side view of a right-side portion of another prior art valve 8 after rotation such that plane “I” is aligned with the page (like the case with FIG. 1B). Here, fixed edge 32 is fully concave from the perspective exterior to valve 8. In the prior art, this fully concave shape was believed to assist in the movement of the leaflet from the open position to the closed position where the leaflet is pushed or draped into the valve interior, as adequate coaptation in the closed state is essential for the proper functioning of the valve.
However, the flat and fully concave shapes of the prior art designs described with respect to FIGS. 1A-1C can lead to a valve with compromised hydrodynamic efficiency due to the fact that the local leaflet length at various heights of the valve is not long enough. This can lead to inadequate valve opening. It can also (or alternatively) lead to local bulging and tightness. The flat or fully concave shapes can both result in localized stress concentrations that, in combination with the aforementioned bulging and tightness, can result in reduced durability and premature failure.
U.S. Pat. No. 6,613,086 (“Moe”) describes other variations in the shape of the support structure (or valve body) for a directly implantable valve. Moe describes “an attachment curve” that is defined as the position where the leaflets are coupled along the inner wall of the support structure. Moe seeks to increase the durability of each leaflet coupled to the support structure by moving the leaflet's point of maximum loaded stress along the attachment curve and away from the location of any stress risers. Moe does this by adjusting the radius of the support structure at different heights along the support structure's axis of flow (see numeral 26 of FIG. 1) and at different radial positions within each cross-sectional plane taken perpendicular to and at different heights along the support structure's axis of flow. As a result, Moe's support structures have substantially non-circular or non-cylindrical inner walls along the attachment curve. These support structures can have significantly asymmetric shapes with substantial surface variations, as evidenced by the bulges 58 and 60 described with respect to FIG. 11 of Moe. Moe's support structures are neither cylindrical nor substantially cylindrical as those terms are used herein.
While trying to reduce the localized stress, Moe's approaches lead to local lengthening of the leaflet at that height in the valve. This local lengthening will lead to an increase in the resistance of the leaflet to open and could compromise the full opening of the valve, leading to local bulging in the leaflet surface. This, in turn, will reduce the hydrodynamic efficiency of the valve and potentially reduce the durability of the valve leaflet.
For these and other reasons, needs exist for improved prosthetic valves.